Allow for custom centroiding in find_peaks

[larrybradley]

This PR allows for custom centroiding in the find_peaks function. Currently, the function will centroid sources with the subpixel keyword (now deprecated) by fitting a 2D Gaussian + constant. As noted in #655, this compound-model fitting method can be slow.

This PR makes centroiding in find_peaks more flexible by allowing the user to pass any callable (e.g. function/class instance) to compute the local centroid (within the cutout/footprint). This PR also adds a new general function called centroid_sources that performs this functionality given initial (x, y) positions.

One small change of behavior is that with subpixel=True the fitted peak value was also returned. To keeps things general, that is no longer the case because the passed centroid function need not perform any fit (e.g. a moment-based centroid).

To get the same centroid values as subpixel=True, one can use the centroid_func keyword with the photutils.centroids.centroid_2dg function.